Metallic orthopedic devices, though extensively used, exhibit problems that are inherent due to the high modulus of the metals used in the devices. A majority of the total joint replacement orthopedic devices implanted to date are of the type comprised of a metallic surface bearing on an ultra-high molecular weight polyethylene cup surface. For hip replacement, for instance, there is a metallic femoral component consisting of a highly polished spherical head attached to a narrow neck which then widens to a tapered shaft design to conform to the contours and to be inserted into the medullary canal. The metal of the shaft is usually a cobalt-chromium-molybdenum alloy or a titanium alloy with mechanical properties and load bearing ability far in excess of that of the bone itself. In a majority of cases, the shaft is attached to the bone by bone cement (e.g., poly methyl methacrylate) which has been packed into the cavity after oversize reaming. The implant shaft is then forced into the curing bone cement and attachment is primarily due to mechanical interlocking.
Failure of hip devices have occurred in many patients. Multiple fractures in the bone cement can lead to the loosening of the implant and the eventual fatigue fracture of the shaft or stem of such a device in vivo. Most important factors contributing to this failure are the design of the stem, the quality and thickness of the bone cement, surgical techniques of bone preparation and cement insertion, imperfections such as metal defects and voids, and improper stress transfer through the surrounding bone by the stem.
One reason for failure of the devices is that resorption of the bone surrounding the proximal portion of the implant stem can lead to cement failure, resulting in the loosening of the stem in the bone. A loose stem cannot function properly and can cause the implant to fail even if the stem itself has not fractured. It is expected that a lower modulus stem will lead to a greater transfer of stresses to the bone in the calcar region and prevent or lessen the degree of resorption of the bone in this region. Based upon this concept, titanium alloy hips have been introduced by several orthopedic manufacturers since it has approximately half of the elastic modulus of cobalt-based alloys. It has been found that reducing stem modulus increases the transfer of load from the stem to the bone. This lack of "stress shielding" effect will lead to a reduction in bone resorption.
Bio-compatible composite materials have been determined to offer the unique advantage of being light weight in construction and high in strength at low modulus values. By far, the most popular composite material for orthopedic implants is one in which reinforcing is due to carbon fibers. A hip prosthesis has been made in the past in which the stem is formed of carbon fiber reinforced carbon and a spherical head is made of aluminum oxide.
Other attempts have been made to investigate the mechanical behavior of fiber reinforced materials specifically for prothesis of different types. Various polymers in combination with carbon or glass fibers were considered in the studies. For instance, it has been shown that quartz and graphite fiber reinforced epoxy composite with strength in the vicinity of 1400 Mpa could be made and proven to be acceptable for implant service. (Musikant, "Quartz and Graphite Filament Reinforced Polymer Composites for Orthopedic Surgical Applications," J. Biomed. Mater. Res. Symp., Vol. 1, pp. 225-235, 1971). In another publication, reports were made of mechanical properties in vitro of a low modulus epoxy carbon fiber composite. (Bradley et al., "Carbon Fiber Reinforced Epoxy as a High Strength Low Modulus Material for Internal Fixation Plates," Biomaterials, Vol. 1, January, 1980). Composite plates were made suitable for internal fixation of fractures. The plates showed superior flexural strength and fatigue properties in comparison with stainless steel plates while having approximately one third the stiffness.
In another publication, namely Litchman et al., "Graphite Reinforced Bone Cement Practical in Orthopedic Surgery", Orthopedic Review, Vol. X, No. 3, March, 1981, a report is made of a 64% increase in strength and a 200% increase in the stiffness of poly methyl methacrylate bone cement due to the addition of just 3% by weight of carbon fiber. While this article suggests such a composite to be of great value as a load bearing implant, other researches have found it to be unsuitable.
Still another publication mentions the use of a carbon fiber-polysulfone composite for making surgical implants but does not specify how the composite is made. This publication is by M. S. Hunt, entitled, "ME1689 An Introduction to the Use of Carbon Fiber Reinforced Composite Materials for Surgical Implants" National Mechanical Engineering Research Institute, Counsel for Scientific and Industrial Research, January 1981, Series No. MEI/8, Reference No. MEI/4054 (Pretoria).
In the present invention, there is disclosed a carbon fiber reinforced polymer composite specifically for use in load bearing orthopedic implants, such as hip joints, knee joints, bone plates and intramedullary rods.